The present invention relates to a fixing device and an image forming apparatus provided therewith.
Conventionally, image forming apparatuses are known that are provided with a fixing device in which a nip region is formed using two rollers, a heating roller (heating section) and a pressure applying roller (pressing section), and passing through that nip region a recording material carrying a toner image thereby fixing it.
In such image forming apparatuses, generally, recording materials of the same size are conveyed at the same position in a direction perpendicular to the conveying direction of the recording materials.
Therefore, when a large number of sheets of the same size are conveyed successively, paper dust and resins contained in the toner are supplied and taken away successively by the sheets in the area in which sheets are conveyed. On the contrary, in the area in which the sheets are not conveyed, paper dust and materials contained in the toner are almost never brought in.
In addition, in the area in which sheets are conveyed, generally, a mechanical load is placed on the heating roller by the step caused by the leading and trailing edges of the sheet and due to undulations on the surface.
Due to the differences in these contacting materials and surface conditions, the surface condition of the heating roller in the area in which the sheets are conveyed was different from the surface condition in the area in which sheets are not conveyed.
Since the state after fixing (such as the glossiness) is affected strongly by the surface condition of the heating roller, if a sheet is conveyed that is larger in size than the sheet that was conveyed previously, due to the above problem, the glossiness or the like were sometimes different in the area in which sheets were being conveyed and in the area in which sheets were not being conveyed bordering on the border between these two areas, and in this case they appeared as image defects (uneven gloss or the like), which was a problem in image forming apparatuses which are required to provide high image quality.
This is because, if the border is clear, the human eye can easily recognize optical differences (attention is being paid in the present invention to uneven gloss), and recognizes as a defect even if the difference in terms of numerical values is small.
Further, since the force of adhesion between the heating roller and the recording material is increased by the molten toner inside the nip region, the recording material gets wound around the heating roller.
In order to solve this problem, measures have been taken such as coating a mold release agent on the heating miler, making the toner contain a low melting point wax which is made to ooze out during fixing, thereby making the heating miler and the recording material easy to separate.
However, because of increasing the speed stable separation was not possible, since the heating roller becomes larger in diameter, the curvature becomes small, and even the speed of the sheet becomes higher. In addition, as a supplementary separating means, a separation claw is being used which mechanically supplements the separation of the recording material when its front edge is made to enter between the recording material and the heating roller.
In order to prevent the image defects (uneven gloss) described above and wrapping of the recording material around the heating roller, a fixing device has been proposed (see Japanese Patent Application Publication No. 2005-351939) wherein, by accommodating inside the same frame the heating roller and a plurality of separation claws that separate the recording material from the heating roller, and by making that frame carry out reciprocating motion in a direction perpendicular to the direction of conveying the recording material, it is ensured that the recording sheet is not conveyed at a fixed position on the heating roller, and also, the plurality of separation claws are made to contact the surface of the heating roller thereby separating the recording material.
However, in the fixing device described in the Japanese Patent Application Publication No. 2005-351939, since the heating roller and a plurality of separation claws are accommodated inside the same frame, at the time that the heating roller is moved in a direction perpendicular to the direction of conveying the recording material, even the plurality of separation claws move together with the heating roller.
Since the separation claws move in this manner, the positional relationship between the sheet and the plurality of separation claws changes, and depending on the dimensions of the sheet and the positions of attaching the separation claws, the separation claws may not be able to butt against the edge part of the recording material that is being conveyed at a predetermined position.
If the separation claws do not butt against the recording material, there was the problem that the recording material is not properly separated from the heating roller, a recording material separation defect (paper jamming) occurs, and as a result the operating rate of the image forming apparatus decreases.
In view of the problems described above, an object of the present invention is to provide fixing device that, in addition to suppressing the generation of image defects (uneven gloss or the like), can prevent separation failure of the recording material, and can suppress the reduction in the operation rate caused by separation failure.